The present invention relates to a magnetic recording medium. More particularly, the present invention relates to a magnetic recording medium having a magnetic recording layer containing surface-modified magnetic particles and/or a surface-modified filler; and a magnetic recording medium having a non-magnetic undercoat layer containing surface-modified non-magnetic particles and a magnetic recording layer on the non-magnetic undercoat layer, containing at least one selected from the group consisting of surface-modified magnetic particles, a surface-modified filler and surface-modified non-magnetic particles, which are capable of inhibiting the production of a cyclohexanone dimer in a magnetic or non-magnetic coating composition upon the production of a magnetic recording medium.
In recent years, miniaturization, lightening, recording-time prolongation, high-density recording and increased memory capacity of magnetic recording and reproducing apparatuses for audio, video or computer, have proceeded more rapidly. With such recent developments, magnetic recording medium used in these apparatuses such as magnetic tapes and magnetic disks have also been required to have a high performance and a high-density recording property.
More specifically, the magnetic recording medium have been required to have a high image definition, a high image quality and high output characteristics, especially excellent frequency characteristics, as well as enhanced storage stability, durability and running property.
In general, the magnetic recording medium has been produced by applying a magnetic coating composition prepared by blending magnetic particles and a binder resin in an organic solvent, onto a non-magnetic base film.
However, it is known that when a ketone-based solvent, especially cyclohexanone, is used as the organic solvent, a cyclohexanone diner is inevitably produced because of high surface activity of the magnetic particles present in the magnetic coating composition. As a result, conventional magnetic recording medium produced by using such a magnetic coating composition containing cyclohexanone as the organic solvent, are unsatisfactory in strength of a magnetic coating film. As to this fact, Japanese Patent Application Laid-Open (KOKAI) No. 59-172562(1984) describes that xe2x80x9c . . . There is a problem that a ketone-based solvent such as especially cyclohexanone tends to be dimerized during storage thereof. As a result, a magnetic layer produced by applying a coating composition containing such a solvent onto a base film tends to suffer from deteriorated strength or bleed-out . . . xe2x80x9d.
For this reason, it has been required to provide magnetic particles and non-magnetic particles exhibiting improved surface properties capable of preventing the production of cyclohexanone dimer in magnetic and non-magnetic coating compositions.
On the other hand, magnetic recording medium have been required to exhibit a still higher performance and, therefore, have an improved strength of coating film as well as enhanced physical properties such as running property.
The running property of magnetic recording medium is ensured by incorporating a fatty acid such as myristic acid and stearic acid (hereinafter referred to merely as xe2x80x9cfatty acidxe2x80x9d) in an amount of usually about 0.5 to about 5% by weight based on the weight of magnetic particles, into a magnetic recording layer generally formed as an upper layer of the magnetic recording medium, and then allowing the fatty acid to be gradually oozed onto the surface of the magnetic recording layer so as to cause the surface of the magnetic recording layer to be slidable.
When the amount of the fatty acid oozed out onto the surface of the magnetic recording layer is too small, it may be difficult to ensure a good running property of the magnetic recording medium. On the other hand, when a too large amount of the fatty acid is added to the magnetic recording layer so as to allow a large amount of the fatty acid to be oozed out onto the surface thereof, the fatty acid is preferentially adsorbed on the surface of the magnetic particles dispersed in the magnetic recording layer, so that the magnetic particles are inhibited from being absorbed in resin. As a result, it may be difficult to disperse the magnetic particles in vehicle resin. Further, an increased amount of the fatty acid as a non-magnetic component causes deterioration in magnetic properties of the magnetic recording medium. In addition, since the fatty acid acts as a plasticizer, there tend to arise additional problems such as deteriorated strength of the magnetic recording medium.
It is also known that magnetic or non-magnetic particles for magnetic recording medium are surface-treated with silane compounds in order to improve surface properties thereof (Japanese Patent Application Laid-Open (KOKAI) Nos. 51-134899(1976), 57-186302(1982), 59-172562(1984), 5-1238(1993), 8-48910(1996) and 8-120118(1996) or the like).
At present, it has been strongly required to provide magnetic particles, filler and non-magnetic particles for magnetic recording medium which are capable of inhibiting the production of cyclohexanone diner in a magnetic or non-magnetic coating composition, and improving a running property of the obtained magnetic recording medium without deterioration in dispersibility thereof. However, magnetic particles, filler and non-magnetic particles for magnetic recording medium which can fulfill these requirements, have not been obtained conventionally.
That is, in Japanese Patent Application Laid-Open (KOKAI) Nos. 51-134899(1976), 57-186302(1982), 59-172562(1984), 5-1238(1993) and 8-48910(1996), it is described that magnetic particles or metal oxide particles are surface-treated with silane compounds. However, since all of the methods are directed to wet-treatment, it may be difficult to sufficiently inhibit the production of cyclohexanone dimer in a magnetic or non-magnetic coating composition and the obtained particles tend to be deteriorated in dispersibility in vehicle, as described in Comparative Examples below.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 8-120118(1996), it is described that an inorganic filler is heat-treated together with organometallic compounds at a temperature of not less than 200xc2x0 C. However, it may be difficult to sufficiently inhibit the production of cyclohexanone dimer in a magnetic or non-magnetic coating composition and the obtained particles tend to be deteriorated in dispersibility in vehicle, as described in Comparative Examples below.
As a result of the present inventors earnest studies, it has been found that by using surface-modified particles obtained by dry-mixing raw particles to be treated with a silane monomer, e.g., mechanically mixing and stirring the raw particles with the silane monomer, or mechanically mixing and stirring the raw particles with the silane compound while spraying the silane compound thereon, a magnetic recording medium are free from the production of cyclohexanone dimer in a magnetic or non-magnetic coating composition, resulting in showing an excellent coating film strength and running durability. The present invention has been attained on the basis of this finding.
An object of the present invention is to provide a magnetic recording medium exhibiting excellent coating film strength and running durability, which is suitable as a high-density magnetic recording medium.
An another object of the present invention is to provide a magnetic recording medium containing surface-modified particles which are capable of not only inhibiting the production of cyclohexanone dimer in a magnetic or non-magnetic coating composition, but also showing an improved dispersibility in vehicle and a reduced fatty acid adsorption.
To accomplish the aims, in a first aspect of the present invention, there are provided a magnetic recording medium comprising:
a non-magnetic base film; and
a magnetic recording layer formed on the non-magnetic base film, which contains a filler, a binder resin and surface-modified magnetic particles having an average particle diameter of 0.01 to 0.7 xcexcm and comprising magnetic particles and an organosilane compound produced from a silane monomer, coated on surface of the magnetic particle by means of a dry-mixing method.
In a second aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film; and
a magnetic recording layer formed on the non-magnetic base film, which contains a binder resin, a surface-modified filler having an average particle diameter of 0.01 to 1.0 xcexcm and comprising a filler and an organosilane compound produced from a silane monomer, coated on surface of the filler by means of a dry-mixing method, and surface-modified magnetic particles having an average particle diameter of 0.01 to 0.7 xcexcm and comprising magnetic particles and an organosilane compound produced from a silane monomer, coated on surface of the magnetic particle by means of a dry-mixing method.
In a third aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film;
a non-magnetic undercoat layer disposed on the surface of the non-magnetic base film, which contains a binder resin, and non-magnetic particles; and
a magnetic recording layer formed on the non-magnetic undercoat layer, which contains a filler, a binder resin and surface-modified magnetic particles having an average particle diameter of 0.01 to 0.7 xcexcm and comprising magnetic particles and an organosilane compound produced from a silane monomer, coated on surface of the magnetic particle by means of a dry-mixing method.
In a fourth aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film;
a non-magnetic undercoat layer disposed on the surface of the non-magnetic base film, which contains a binder resin, and surface-modified non-magnetic particles having an average particle diameter of 0.01 to 0.5 xcexcm and comprising acicular non-magnetic particles and an organosilane produced from a silane monomer, coated on surface of the acicular non-magnetic particles by means of a dry-mixing method; and
a magnetic recording layer formed on the non-magnetic undercoat layer, which contains a filler, a binder resin and surface-modified magnetic particles having an average particle diameter of 0.01 to 0.7 xcexcm and comprising magnetic particles and an organosilane compound produced from a silane monomer, coated on surface of the magnetic particle by means of a dry-mixing method.
In a fifth aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film; and
a magnetic recording layer formed on the non-magnetic base film, which contains magnetic particles, a binder resin, and a surface-modified filler having an average particle diameter of 0.01 to 1.0 xcexcm and comprising a filler and an organosilane compound produced from a silane monomer, coated on the surface of the filler by means of a dry-mixing method.
In a sixth aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film;
a non-magnetic undercoat layer disposed on the surface of the non-magnetic base film, which contains a binder resin and surface-modified non-magnetic particles having an average particle diameter of 0.01 to 0.5 xcexcm and comprising acicular non-magnetic particles and an organosilane compound produced from a silane monomer, coated on surface of the acicular non-magnetic particle by means of a dry-mixing method; and
a magnetic recording layer formed on the non-magnetic undercoat layer, which contains magnetic particles, a binder resin, and a surface-modified filler having an average particle diameter of 0.01 to 1.0 xcexcm and comprising a filler and an organosilane compound produced from a silane monomer, coated on the surface of the filler by means of a dry-mixing method.
In a seventh aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film;
a non-magnetic undercoat layer disposed on the surface of the non-magnetic base film, which contains a binder resin, and surface-modified non-magnetic particles having an average particle diameter of 0.01 to 0.5 xcexcm and comprising acicular non-magnetic particles and organosilane compound produced from a silane monomer, coated on surface of the acicular non-magnetic particle by means of a dry-mixing method; and
a magnetic recording layer formed on the non-magnetic undercoat layer, which contains a binder resin, a surface-modified filler having an average particle diameter of 0.01 to 1.0 xcexcm and comprising a filler and an organosilane compound produced from a silane monomer, coated on surface of the filler by means of a dry-mixing method, and surface-modified magnetic particles having an average particle diameter of 0.01 to 0.7 xcexcm and comprising magnetic particles and an organosilane compound produced from a silane monomer, coated on surface of the magnetic particle by means of a dry-mixing method.
The present invention will be described in detail below.
The surface-modified particles used in the present invention comprise raw particles to be treated such as magnetic particles, filler or non-magnetic particles, and an organosilane compound produced from a silane monomer, coated onto the surface of these raw particles.
More specifically, the surface-modified particles used in the present invention are surface-modified magnetic particles, surface-modified filler or surface-modified non-magnetic particles, and are produced by dry-mixing the magnetic particles, filler or non-magnetic particles with the silane monomer (hereinafter, the surface-modified magnetic particles, the surface-modified filler or the surface-modified magnetic non-magnetic particles are totally referred to as xe2x80x9csurface-modified particlesxe2x80x9d, and the magnetic particles, filler or non-magnetic particles to be treated are totally referred to as xe2x80x9craw particlesxe2x80x9d).
First, the surface-modified particles used in the present invention are described.
As the magnetic particles to be treated used in the present invention, there may be exemplified (i) magnetic acicular particles, e.g., cobalt-coated magnetic acicular iron oxide particles obtained by coating magnetic acicular iron oxide particles such as acicular magnetic particles (xcex3-Fe2O3) and acicular magnetite particles (FeOxxc2x7Fe2O3, 0 less than xxe2x89xa61) with Co, or Co and Fe; cobalt-coated magnetic acicular iron oxide particles obtained by incorporating elements other than Fe such as Co, Al, Ni, P, Zn, Si, B and rare earth metals into the above cobalt-coated magnetic acicular iron oxide particles; magnetic acicular metal particles containing iron as a main component; and magnetic acicular iron alloy particles containing elements other than Fe such as Co, Al, Ni, P, Zn, Si, B and rare earth metals; (ii) plate-shaped magnetic particles, e.g., plate-shaped magnetoplumbite-type ferrite particles containing Ba, Sr or Baxe2x80x94Sr; and plate-shaped magnetoplumbite-type ferrite particles obtained by incorporating one or more coercive force reducing agents selected from the group consisting of divalent and tetravalent metals such as Co, Ni, Zn, Mn, Mg, Ti, Sn, Zr, Nb, Cu and Mo, into the above ferrite particles; and the like. Meanwhile, the magnetic particles used in the present invention may include either the acicular particles, the plate-shaped particles or both thereof unless otherwise specified. The acicular particles may include needle-like particles, spindle-shaped particles, rice ball-shaped particles or the like; plate-shaped particles.
In the consideration of the high-density recording of the magnetic recording medium, as the magnetic particles used as raw particles in the present invention, there are preferred magnetic acicular metal particles containing iron as a main component, and magnetic acicular iron alloy particles containing elements other than Fe such as Co, Al, Ni, P, Zn, Si, B and rare earth metals.
Specifically, the magnetic acicular iron-based alloy particles comprising (i) iron and Al; (ii) iron, Co and Al, (iii) iron, Al and at least one rare-earth metal such as Nd, La and Y, or (iv) iron, Co, Al and at least one rare-earth metal such as Nd, La and Y is more preferable from the point of the durability of the magnetic recording medium. The amount of cobalt contained in the magnetic particles is usually 0.5 to 50 atomic % (calculated as Co); the amount of aluminum contained in the magnetic particles is usually 0.5 to 30 atomic % (calculated as Al); the amount of rare earth metals contained in the magnetic particles is usually 0.5 to 30 atomic % (calculated as the rare earth element).
More specifically, the magnetic acicular iron-based alloy particles may be exemplified as follows.
1) Magnetic acicular iron-based alloy particles comprises iron; and cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular iron-based alloy particles.
2) Magnetic acicular iron-based alloy particles comprise iron; and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular iron-based alloy particles.
3) Magnetic acicular iron-based alloy particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular iron-based alloy particles; and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular iron-based alloy particles.
4) Magnetic acicular iron-based alloy particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular iron-based alloy particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles.
5) Magnetic acicular iron-based alloy particles comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular iron-based alloy particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles.
6) Magnetic acicular iron-based alloy particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular iron-based alloy particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular iron-based alloy particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles.
7) Magnetic acicular iron-based alloy particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular iron-based alloy particles; and at least one selected from the group consisting of Ni, P, Si, Zn, fi, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles.
8) Magnetic acicular iron-based alloy particles comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular iron-based alloy particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles.
9) Magnetic acicular iron-based alloy particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular iron-based alloy particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular iron-based alloy particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles.
10) Magnetic acicular iron-based alloy particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular iron-based alloy particles; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles.
11) Magnetic acicular iron-based alloy particles comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular iron-based alloy particles; at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles.
12) Magnetic acicular iron-based alloy particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular iron-based alloy particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular iron-based alloy particles; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular iron-based alloy particles.
The iron content in the particles is the balance, and is preferably 50 to 99% by weight, more preferably 60 to 95% by weight (calculated as Fe) based on the weight of the magnetic acicular metal particles containing iron as a main component or the magnetic acicular iron-based alloy particles.
The acicular magnetic particles as the raw particles have an average particle diameter (average major axis diameter) of usually 0.01 to 0.70 xcexcm, preferably 0.02 to 0.60 xcexcm, more preferably 0.03 to 0.50 xcexcm; and an aspect ratio (ratio of average major axis diameter to average minor axis diameter; hereinafter referred to merely as xe2x80x9caspect ratioxe2x80x9d) of preferably 2.0:1 to 20.0:1, more preferably 2.5:1 to 18.0:1, still more preferably 3.0:1 to 15.0:1.
The plate-shaped magnetic particles as the raw particles have an average particle diameter (average plate surface diameter) of usually 0.01 to 0.20 xcexcm, preferably 0.02 to 0.20 xcexcm, more preferably 0.03 to 0.20 xcexcm; and an plate ratio (ratio of average plate surface diameter to average thickness; hereinafter referred to merely as xe2x80x9cplate ratioxe2x80x9d) of preferably 2.0:1 to 20.0:1, more preferably 2.5:1 to 15.0:1, still more preferably 3.0:1 to 10.0:1.
The magnetic particles as the raw particles have a geometrical standard deviation value of particle diameter of preferably not more than 2.00, more preferably not more than 1.90, still more preferably not more than 1.80; and a BET specific surface area value of preferably 15 to 200 m2/g, more preferably 20 to 150 m2/g, still more preferably 25 to 100 m2/g.
The magnetic particles as the raw particles have a cyclohexanone dimer production activity of usually 10 to 500 mg/g; a resin adsorption of usually not more than 60%; and a fatty acid adsorption of usually 18 to 50 mg/g.
As to magnetic properties of the magnetic particles as the raw particles, the coercive force value thereof is preferably 39.8 to 318.3 kA/m (500 to 4,000 Oe), more preferably 43.8 to 318.3 kA/m (550 to 4,000 Oe); and the saturation magnetization value thereof is preferably 35 to 170 Am2/kg (35 to 170 emu/g), more preferably 40 to 170 Am2/kg (40 to 170 emu/g).
More specifically, in the case where the magnetic particles as the raw particles are cobalt-coated acicular magnetic iron oxide particles, the coercive force value thereof is preferably 39.8 to 135.3 kA/m (500 to 1,700 Oe); and the saturation magnetization value thereof is preferably 60 to 90 Am2/kg (60 to 90 emu/g). In the case where the magnetic particles as the raw particles are magnetic acicular metal particles containing iron as a main component or magnetic acicular iron alloy particles, the coercive force value thereof is preferably 63.7 to 278.5 kA/m (800 to 3,500 Oe); and the saturation magnetization value thereof is preferably 90 to 170 Am2/kg (90 to 170 emu/g). In the case where the magnetic particles as the raw particles are plate-shaped magnetoplumbite-type magnetic particles, the coercive force value thereof is preferably 39.8 to 318.3 kA/m (500 to 4,000 Oe); and the saturation magnetization value thereof is preferably 35 to 70 Am2/kg (35 to 70 emu/g).
The organosilane compound is a compound produced from the silane monomer. As the silane monomer, there may be exemplified an alkoxysilane represented by the formula (I):
R1aSiX4-axe2x80x83xe2x80x83(I)
wherein R1 is C6H5xe2x80x94, (CH3)2CHCH2xe2x80x94 or n-CbH2b+1xe2x80x94 (wherein b is an integer of 1 to 18); X is CH3Oxe2x80x94 or C2H5Oxe2x80x94; and a is an integer of 0 to 3.
Examples of the alkoxysilane may include methyltriethoxysilane, dimethyldiethoxysilane, phenyltriethoxysilane, diphenyldiethoxysilane, dimethyldimethoxysilane, methyltirmethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, isobutyltrimethoxysilane, decyltrimethoxysilane or the like.
Among these silane monomers, in the consideration of good coating effect onto the raw particles, methyltriethoxysilane, methyltirmethoxysilane, dimethyldimethoxysilane, isobutyltrimethoxysilane and phenyltriethoxysilane are preferred, and methyltriethoxysilane, methyltirmethoxysilane and phenyltriethoxysilane are more preferred.
When the acicular magnetic particles are used as the raw particles, the obtained surface-modified magnetic particles used in the present invention have an average particle diameter (average major axis diameter) of usually 0.01 to 0.70 xcexcm, preferably 0.02 to 0.60 xcexcm, more preferably 0.03 to 0.50 xcexcm; and an aspect ratio of preferably 2.0:1 to 20.0:1, more preferably 2.5:1 to 18.0:1, still more preferably 3.0:1 to 15.0:1.
When the average particle diameter of the surface-modified magnetic particles produced by using the acicular magnetic particles as the raw particles, is more than 0.70 xcexcm, the obtained surface-modified magnetic particles become coarse, so that a magnetic recording layer formed by using such coarse surface-modified magnetic particles tends to be deteriorated in surface smoothness. On the other hand, when the average major axis diameter of the surface-modified magnetic particles is less than 0.01 xcexcm, such particles tend to be agglomerated together because of the increase of the intermolecular force therebetween due to fine particles, resulting in poor dispersibility in vehicle upon production of a magnetic coating composition.
When the aspect ratio of the surface-modified magnetic particles produced by using the acicular magnetic particles as the raw particles is more than 20.0:1, such particles tend to be entangled together, resulting in poor dispersibility in vehicle as well as increased viscosity upon production of a magnetic coating composition. When the aspect ratio is less than 2.0:1, the obtained magnetic recording medium tends to be deteriorated in coating film strength.
When the plate-shaped magnetic particles are used as the raw particles, the obtained surface-modified magnetic particles used in the present invention have an average particle diameter (average plate surface diameter) of usually 0.01 to 0.20 xcexcm, preferably 0.02 to 0.20 xcexcm, more preferably 0.03 to 0.20 xcexcm; and a plate ratio of preferably 2.0:1 to 20.0:1, more preferably 2.5:1 to 15.0:1, still more preferably 3.0:1 to 10.0:1.
When the average particle diameter of the surface-modified magnetic particles produced by using the plate-shaped magnetic particles as the raw particles, is more than 0.20 xcexcm, the obtained surface-modified magnetic particles become coarse, so that a magnetic recording layer formed by using such coarse surface-modified magnetic particles tends to be deteriorated in surface smoothness. On the other hand, when the average particle diameter of the surface-modified magnetic particles is less than 0.01 xcexcm, such particles tend to be agglomerated together because of the increase of the intermolecular force therebetween due to fine particles, resulting in poor dispersibility in vehicle upon production of a magnetic coating composition.
When the plate ratio of the surface-modified magnetic particles produced by using the plate-shaped magnetic particles is more than 20.0:1, such particles tend to suffer from stacking, resulting in poor dispersibility in vehicle as well as increased viscosity upon production of a magnetic coating composition. When the plate ratio is less than 2.0:1, the obtained magnetic recording medium tends to be deteriorated in coating film strength.
The surface-modified magnetic particles have a geometrical standard deviation value of particle diameters of preferably not more than 2.00. When the geometrical standard deviation value is more than 2.00, the obtained coating film tends to be deteriorated in surface smoothness because of existence of coarse particles. In the consideration of good surface smoothness of the obtained coating film, the geometrical standard deviation value of the surface-modified magnetic particles is more preferably not more than 1.90, still more preferably not more than 1.80. In the consideration of industrial productivity, the lower limit of the geometrical standard deviation value of particle diameters of the surface-modified magnetic particles is 1.01. Namely, it may be difficult to industrially produce such particles having a geometrical standard deviation value of less than 1.01.
The surface-modified magnetic particles have a BET specific surface area value of preferably 15 to 200 m2/g, more preferably 20 to 150 m2/g, still more preferably 25 to 100 m2/g. When the BET specific surface area value is less than 15 m2/g, the obtained surface-modified magnetic particles become coarse, or suffer from sintering therebetween, so that a magnetic recording layer produced by using such particles tends to be deteriorated in surface smoothness. When the BET specific surface area value is more than 200 m2/g, such particles tend to be agglomerated together by the increase of intermolecular force therebetween due to fine particles, resulting in poor dispersibility in vehicle upon production of a magnetic coating composition.
The surface-modified magnetic particles used in the present invention exhibit a cyclohexanone dimer production activity of usually not more than 50 mg/g, preferably not more than 40 mg/g. When the cyclohexanone dimer production activity is more than 50 mg/g, the obtained magnetic recording medium tends to be deteriorated in strength.
The surface-modified magnetic particles used in the present invention have a resin adsorption of usually not less than 70%, preferably not less than 72%, more preferably not less than 74%.
The surface-modified magnetic particles used in the present invention have a fatty acid adsorption of usually not more than 16 mg/g, preferably 0.5 to 15 mg/g. When the fatty acid adsorption is more than 16 mg/g, the amount of fatty acid absorbed onto the surface-modified magnetic particles becomes large, resulting in increased amount of the fatty acid oozed out onto the surface of the magnetic recording layer. As a result, it may be difficult to ensure a good running property of the obtained magnetic recording medium.
The surface-modified magnetic particles used in the present invention can still maintain the above magnetic properties of the magnetic particles as the raw particles. More specifically, the surface-modified magnetic particles used in the present invention have a coercive force value of preferably 39.8 to 318.3 kA/m (500 to 4,000 Oe), more preferably 43.8 to 318.3 kA/m (550 to 4,000 Oe); and a saturation magnetization value of preferably 35 to 170 AM2/kg (35 to 170 emu/g), more preferably 40 to 170 Am2/kg (40 to 170 emu/g).
In the surface-modified magnetic particles, the amount of the organosilane compound produced from the silane monomer onto the raw particles is usually 0.04 to 15% by weight, preferably 0.06 to 10% by weight, more preferably 0.08 to 5% by weight (calculated as Si) based on the weight of the surface-modified magnetic particles. When the amount of the organosilane compound produced from the silane monomer is less than 0.04% by weight, it may be difficult to inhibit the production of cyclohexanone dimer. When the amount of the organosilane compound produced from the silane monomer is more than 15% by weight, since the effect of inhibiting the production of cyclohexanone dimer is already saturated, it is unnecessary and meaningless to coat the raw particles with such a large amount of the organosilane compound produced from the silane monomer.
Next, the surface-modified filler used in the present invention is described.
Examples of the filler used as the raw particles in the present invention may include particles of hematite, alumina, zirconium oxide, cerium oxide, chromium oxide or the like.
The filler as the raw particles may have various shapes. For example, there may be used granular particles having a spherical shape, a granular shape, an octahedral shape, a hexahedral shape, a polyhedral shape or the like; or acicular particles having an acicular shape, a spindle shape, a rice-ball shape or the like. Among these particles, in the consideration of dispersibility in vehicle, granular particles are preferred.
As to the particle size of the filler as the raw particles, in case of granular particles, the average particle diameter thereof is usually 0.01 to 1.0 xcexcm, preferably 0.02 to 0.90 xcexcm, more preferably 0.03 to 0.80 xcexcm.
In case of acicular particles, the average particle diameter (average major axis diameter) thereof is usually 0.01 to 1.00 xcexcm, preferably 0.02 to 0.90 xcexcm, more preferably 0.03 to 0.80 xcexcm, and the aspect ratio thereof is usually 2.0:1 to 20.0:1, preferably 2.5:1 to 15.0:1, more preferably 3.0:1 to 10.0:1.
The filler as the raw particles has a geometrical standard deviation value of particle diameters of preferably not more than 1.80, more preferably not more than 1.70, still more preferably not more than 1.60; and a BET specific surface area value of preferably 1.0 to 200 m2/g, more preferably 1.5 to 150 m2/g, still more preferably 2.0 to 100 m2/g.
The filler as the raw particles has a cyclohexanone dimer production activity of usually 50 to 150 mg/g; a resin adsorption of usually not more than 60%; and a fatty acid adsorption of usually 8 to 50 mg/g.
As to the particle size of the surface-modified filler used in the present invention, the average particle diameter thereof is usually 0.01 to 1.0 xcexcm, preferably 0.02 to 0.9 xcexcm, more preferably 0.03 to 0.8 xcexcm.
More specifically, when the granular filler is used as the raw particle, the obtained surface-modified filler has an average particle diameter of usually 0.01 to 1.0 xcexcm, preferably 0.02 to 0.9 xcexcm, more preferably 0.03 to 0.8 xcexcm.
When the acicular filler is used as the raw particles, the obtained surface-modified filler has an average particle diameter (average major axis diameter) of usually 0.01 to 1.0 xcexcm, preferably 0.02 to 0.9 xcexcm, more preferably 0.03 to 0.8 xcexcm; and an aspect ratio of usually 2.0:1 to 20.0:1, preferably 2.5:1 to 15.0:1, more preferably 3.0:1 to 10.0:1.
When the average particle diameter of the surface-modified filler is less than 0.01 xcexcm, such particles tend to be agglomerated together because of the increase of intermolecular force therebetween due to fine particles, resulting in poor dispersibility in vehicle upon production of a magnetic coating composition. As a result, the obtained magnetic recording medium tends to be deteriorated in durability and surface smoothness. On the other hand, when the average particle diameter of the surface-modified filler is more than 1.0 xcexcm, the obtained surface-modified filler has a too large particle size, so that a magnetic recording layer formed by using such a large surface-modified filler tends to be deteriorated in surface smoothness.
When the aspect ratio of the surface-modified filler having an acicular particle shape is more than 20.0:1, such particles tend to be entangled together, so that it may be difficult to uniformly disperse the surface-modified filler in vehicle upon the production of a magnetic coating composition. As a result, it may also be difficult to obtain a magnetic recording medium exhibiting excellent durability and surface smoothness.
The surface-modified filler has a geometrical standard deviation value of particle diameters of preferably not more than 1.80. When the geometrical standard deviation value is more than 1.80, the surface smoothness of a coating film formed by using such a surface-modified filler tends to be deteriorated because of existence of coarse particles. In the consideration of good surface smoothness of the obtained coating film, the geometrical standard deviation value of the surface-modified filler is more preferably not more than 1.70, still more preferably not more than 1.60. In the consideration of industrial productivity, the lower limit of the geometrical standard deviation value of particle diameters of the surface-modified filler is 1.01. Namely, it may be difficult to industrially produce such a filler having a geometrical standard deviation value of less than 1.01.
The surface-modified filler has a BET specific surface area value of preferably 1 to 200 m2/g, more preferably 1.5 to 150 m2/g, still more preferably 2 to 100 m2/g. When the BET specific surface area value is less than 1 m2/g, the obtained surface-modified filler may become coarse, or may suffer from sintering therebetween, so that a magnetic recording medium produced by using such a filler tends to be deteriorated in surface smoothness of a coating film. When the BET specific surface area value is more than 200 m2/g, such a filler tends to be agglomerated together by the increase of intermolecular force therebetween due to fine particles, resulting in poor dispersibility in vehicle upon production of a magnetic coating composition.
The surface-modified filler used in the present invention exhibits a cyclohexanone diner production activity of usually not more than 40 mg/g, preferably not more than 30 mg/g.
The surface-modified filler used in the present invention has a resin adsorption of usually not less than 70%, preferably not less than 72%, more preferably not less than 74%.
The surface-modified filler used in the present invention has a fatty acid adsorption of usually not more than 7 mg/g, preferably 0.5 to 6 mg/g.
In the surface-modified filler used in the present invention, the amount of the organosilane compound produced from the silane monomer onto the filler as the raw particles is usually 0.04 to 15% by weight, preferably 0.06 to 10% by weight, more preferably 0.08 to 5% by weight (calculated as Si) based on the weight of the obtained surface-modified filler. When the amount of the organosilane compound produced from the silane monomer is less than 0.04% by weight, it may be difficult to inhibit the production of cyclohexanone diner. When the amount of organosilane compound produced from the silane monomer is more than 15% by weight, since the effect of inhibiting the production of cyclohexanone dimer is already saturated, it is unnecessary and meaningless to coat the filler with such a large amount of the organosilane compound produced from the silane monomer.
Next, the surface-modified non-magnetic particles used in the present invention are described.
Specific examples of the non-magnetic particles may include hematite particles, iron oxide hydroxide particles, titanium oxide particles, zinc oxide particles, tin oxide particles, tungsten oxide particles, silicon dioxide particles, xcex1-alumina particles, xcex2-alumina particles, xcex3-alumina particles, chromium oxide particles, cerium oxide particles, silicon carbide particles, titanium carbide particles, silicon nitride particles, boron nitride particles, calcium carbonate particles, barium carbonate particles, magnesium carbonate particles, strontium carbonate particles, calcium sulfate particles, barium sulfate particles, molybdenum disulfide particles, barium titanate particles or the like. These non-magnetic particles may be used alone or in combination of any two or more thereof. Among these non-magnetic particles, hematite particles, iron oxide hydroxide particles and titanium oxide particles, etc., are preferred.
The non-magnetic particles as the raw particles have an acicular shape. The xe2x80x9cacicularxe2x80x9d shape used herein is intended to involve in addition to literally an acicular shape, a spindle shape, a rice-grain shape or the like.
The non-magnetic particles as the raw particles have an average major axis diameter of usually 0.01 to 0.50 xcexcm, preferably 0.02 to 0.40 xcexcm, more preferably 0.03 to 0.30 xcexcm; an aspect ratio of usually 2.0:1 to 20.0:1, preferably 2.5:1 to 18.0:1, more preferably 3.0:1 to 15.0:1; a geometrical standard deviation value of particle diameters of preferably not more than 1.50, more preferably not more than 1.48, still more preferably not more than 1.45; and a BET specific surface area value of preferably 35 to 250 m2/g, more preferably 38 to 200 m2/g, still more preferably 40 to 180 m2/g.
The non-magnetic particles as the raw particles have a cyclohexanone dimer production activity of usually 150 to 300 mg/g; a resin adsorption of usually not more than 60%; and a fatty acid adsorption of usually 16 to 60 mg/g.
The surface-modified non-magnetic particles used in the present invention have an average major axis diameter of usually 0.01 to 0.50 xcexcm, preferably 0.02 to 0.40 xcexcm, more preferably 0.03 to 0.30 xcexcm.
When the average major axis diameter is more than 0.50 xcexcm, the obtained surface-modified non-magnetic particles become coarse, so that the non-magnetic undercoat layer formed by using such coarse surface-modified non-magnetic particles tends to be deteriorated in surface smoothness. On the other hand, when the average particle diameter is less than 0.01 xcexcm, such particles tend to be agglomerated together because of the increase of intermolecular force therebetween due to fine particles, resulting in poor dispersibility in vehicle upon production of a non-magnetic coating composition.
The surface-modified non-magnetic particles used in the present invention have an aspect ratio of usually 2.0:1 to 20.0:1, preferably 2.5:1 to 18.0:1, more preferably 3.0:1 to 15.0:1.
When the aspect ratio of the surface-modified non-magnetic particles is more than 20.0:1, such particles tend to be entangled together, resulting in poor dispersibility in vehicle as well as increased viscosity upon the production of a non-magnetic coating composition. When the aspect ratio of the surface-modified non-magnetic particles is less than 2.0:1, the non-magnetic undercoat layer formed by using such surface-modified non-magnetic particles tends to be deteriorated in strength.
The surface-modified non-magnetic particles have a geometrical standard deviation value of particle diameters of preferably not more than 1.50. When the geometrical standard deviation value is more than 1.50, the coating film formed by using such surface-modified non-magnetic particles tends to be deteriorated in surface smoothness because of existence of coarse particles. In the consideration of good surface smoothness of the obtained coating film, the geometrical standard deviation value of particle diameters of the surface-modified non-magnetic particles is more preferably not more than 1.48, still more preferably not more than 1.45. In the consideration of industrial productivity, the lower limit of the geometrical standard deviation value of particle diameters of the surface-modified non-magnetic particles is 1.01. Namely, it may be difficult to industrially produce such surface-modified non-magnetic particles having a geometrical standard deviation value of less than 1.01.
The surface-modified non-magnetic particles have a BET specific surface area value of preferably 35 to 250 m2/g, more preferably 38 to 200 m2/g, still more preferably 40 to 180 m2/g. When the BET specific surface area value is less than 35 m2/g, the obtained surface-modified non-magnetic particles become coarse, or suffer from sintering therebetween, so that the non-magnetic undercoat layer formed by using such surface-modified non-magnetic particles tends to be deteriorated in surface smoothness. When the BET specific surface area value is more than 250 m2/g, such non-magnetic particles tend to be agglomerated together by the increase of intermolecular force therebetween due to fine particles, resulting in poor dispersibility in vehicle upon production of a non-magnetic coating composition.
The surface-modified non-magnetic particles used in the present invention have a cyclohexanone dimer production activity of usually not more than 50 mg/g, preferably not more than 40 mg/g. When the cyclohexanone dimer production activity of the surface-modified non-magnetic particles is more than 50 mg/g, the obtained magnetic recording medium tends to be deteriorated in strength.
The surface-modified non-magnetic particles used in the present invention have a resin adsorption of usually not less than 70%, preferably not less than 72%, more preferably not less than 74%.
The surface-modified non-magnetic particles used in the present invention have a fatty acid adsorption of usually not more than 15 mg/g, preferably 0.5 to 10 mg/g.
When the fatty acid adsorption is less than 0.5 mg/g, it may be difficult to adequately control the amount of the fatty acid oozed out onto the surface of the magnetic recording layer because of a too small amount of the fatty acid absorbed into the surface-modified non-magnetic particles, thereby failing to obtain magnetic recording medium having a sufficient running property. When the fatty acid adsorption is more than 15 mg/g, the amount of the fatty acid oozed out onto the surface of the magnetic recording layer becomes too small since a large amount of the fatty acid is absorbed into the surface-modified non-magnetic particles, so that it may be difficult to ensure a good running property of the obtained magnetic recording medium.
In the surface-modified non-magnetic particles, the amount of the organosilane compound produced from the silane monomer onto the raw particles is usually 0.04 to 15% by weight, preferably 0.06 to 10% by weight, more preferably 0.08 to 5% by weight (calculated as Si) based on the weight of the surface-modified non-magnetic particles.
When the amount of the organosilane compound produced from the silane monomer is less than 0.04% by weight, it may be difficult to inhibit the production of cyclohexanone dimer. It is unnecessary and meaningless to adhere the organosilane compound produced from the silane monomer in an amount of more than 15% by weight since the effect of inhibiting the production of cyclohexanone dimer is already saturated.
Upon the production of the surface-modified particles used in the present invention, the surface of the raw particles may be previously coated, if required, with at least one coating material selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon. The surface-modified particles produced by using such coated raw particles can exhibit a more excellent dispersibility in vehicle as compared to those produced by using uncoated raw particles.
The total amount of the coating material disposed between the surface of the raw particles and the organosilane compound layer is preferably 0.01 to 20% by weight (calculated as Al, SiO2 or a sum of Al and SiO2) based on the weight of the coated raw particles.
When the amount of the coating material is less than 0.01% by weight, it may be difficult to obtain the effect of enhancing the dispersibility. Since the amount of the coating material up to 20% by weight is sufficient to attain the effect of enhancing the dispersibility, it is unnecessary and meaningless to coat the raw particles with the coating material in an amount of more than 20% by weight.
The surface-modified particles produced by using the raw particles coated with the coating material, have the substantially same particle size, geometrical standard deviation value, BET specific surface area value, magnetic properties, cyclohexanone dimer production ability and fatty acid adsorption as those of the surface-modified particles produced by using uncoated raw particles according to the present invention.
The surface-modified particles can also be enhanced in resin adsorption by coating the surface of the raw particles with the above coating material. More specifically, the surface-modified particles produced by using the coated raw particles can exhibit a resin adsorption of preferably not less than 74%, more preferably not less than 76%.
Next, the magnetic recording medium of the present invention is described.
The magnetic recording medium of the present invention comprises a non-magnetic base film, and a magnetic recording layer formed on the non-magnetic base film, which contains the surface-modified magnetic particles, an ordinary filler (i.e., surface-unmodified filler; hereinafter referred to merely as xe2x80x9cordinary fillerxe2x80x9d) and a binder resin, or contains magnetic particles, the surface-modified filler and a binder resin. In addition, when the surface-modified magnetic particles are used together with the surface-modified filler, the obtained magnetic recording medium can exhibit a more excellent effect of inhibiting the production of cyclohexanone dimer.
As the non-magnetic base film, the following materials which are at present generally used for the production of a magnetic recording medium are usable as a raw material: a synthetic resin such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamideimide and polyimide; foil and plate of a metal such as aluminum and stainless steel; and various kinds of paper. The thickness of the non-magnetic base film varies depending upon the material, but it is usually about 1.0 to 300 xcexcm, preferably 2.0 to 200 xcexcm.
In the case of a magnetic disc, polyethylene terephthalate is usually used as the non-magnetic base film. The thickness thereof is usually 50 to 300 xcexcm, preferably 60 to 200 xcexcm. In the case of a magnetic tape, when polyethylene terephthalate is used as the base film, the thickness thereof is usually 3 to 100 xcexcm, preferably 4 to 20 xcexcm. When polyethylene naphthalate is used, the thickness thereof is usually 3 to 50 xcexcm preferably 4 to 20 xcexcm. When polyamide is used, the thickness thereof is usually 2 to 10 xcexcm, preferably 3 to 7 xcexcm.
As the binder resin used in the present invention, the following resins which are at present generally used for the production of a magnetic recording medium are usable: vinyl chloride-vinyl acetate copolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane elastomer, butadiene-acrylonitrile copolymer, polyvinyl butyral, cellulose derivative such as nitrocellulose, polyester resin, synthetic rubber resin such as polybutadiene, epoxy resin, polyamide resin, polyisocyanate, electron radiation curing acryl urethane resin and mixtures thereof.
Each of these resin binders may contain a functional group such as xe2x80x94OH, xe2x80x94COOH, xe2x80x94SO3M, xe2x80x94OPO2M2 and xe2x80x94NH2, wherein M represents H, Na or K. In the consideration of good dispersibility of the surface-modified magnetic particles or the ordinary magnetic particles and the surface-modified filler or the ordinary filler in vehicle upon production of a magnetic coating composition, the use of such a binder resins having as a functional group xe2x80x94COOH or xe2x80x94SO3M is preferred.
The thickness of the magnetic recording layer obtained by applying the magnetic coating composition on the surface of the non-magnetic base film and dried, is usually in the range of 0.01 to 5.0 xcexcm. If the thickness is less than 0.01 xcexcm, uniform coating may be difficult, so that unfavorable phenomenon such as unevenness on the coating surface is observed. On the other hand, when the thickness exceeds 5.0 xcexcm, it may be difficult to obtain desired electromagnetic performance due to an influence of diamagnetism.
The amount of the binder resin contained in the magnetic recording layer is usually 5 to 100 parts by weight, preferably 6 to 50 parts by weight based on 100 parts by weight of the surface-modified magnetic particles or the ordinary magnetic particles contained in the magnetic recording layer.
When the amount of the binder resin is more than 100 parts by weight, the amount of the surface-modified magnetic particles or the ordinary magnetic particles filled in the magnetic recording layer becomes comparatively too small, resulting in deteriorated magnetic properties thereof. When the amount of the binder resin is less than 5 parts by weight, the surface-modified magnetic particles or the ordinary magnetic particles are not sufficiently dispersed in the magnetic coating composition because of a too small amount of the binder resin as compared to the amount of the particles, so that it may be difficult to form a coating film having a sufficiently smooth surface. Also, since the surface-modified magnetic particles or the ordinary magnetic particles cannot be sufficiently bound together by the binder resin, the obtained coating film tends to become brittle.
The amount of the surface-modified filler or the ordinary filler contained in the magnetic recording layer is usually 1 to 30 parts by weight, preferably 3 to 25 parts by weight based on 100 parts by weight of the surface-modified magnetic particles or the ordinary magnetic particles contained in the magnetic recording layer.
When the amount of the surface-modified filler or the ordinary filler is less than 1 part by weight, the obtained magnetic recording medium tends to be deteriorated in durability because of a too small amount of the surface-modified filler or the ordinary filler contained in the magnetic recording layer. When the amount of the surface-modified filler or the filler is more than 30 parts by weight, although the obtained magnetic recording medium exhibits a sufficient durability, the amount of non-magnetic components contained in the magnetic recording layer is increased, which becomes disadvantageous for producing a high-density recording magnetic recording medium.
Meanwhile, the magnetic recording layer may contain, if required, known additives for an ordinary magnetic recording medium such as lubricants, abrasives and antistatic agents in an amount of about 0.1 to about 50 parts by weight based on 100 parts by weight of the binder resin.
The magnetic recording medium using the surface-modified magnetic particles and the ordinary filler according to the present invention has a coercive force value of preferably 39.8 to 318.3 kA/m (500 to 4,000 Oe), more preferably 43.8 to 318.3 kA/m (550 to 4,000 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm; hereinafter referred to merely as xe2x80x9csquarenessxe2x80x9d) of preferably 0.85 to 0.95, more preferably 0.86 to 0.95; a gloss of coating film of preferably 165 to 300%, more preferably 170 to 300%; a surface roughness Ra of coating film of preferably not more than 10.5 nm, more preferably 2.0 to 10.0 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of preferably 126 to 160, more preferably 127 to 160; a running durability of preferably not less than 22 minutes, more preferably not less than 23 minutes; and a scratch resistance of preferably Rank A or B, more preferably Rank A as measured by the below-mentioned evaluation method. In addition, the cyclohexanone diner extraction amount is preferably not more than 50 mg/m2, more preferably not more than 40 mg/m2, still more preferably not more than 30 mg/m2 as measured by the below-mentioned evaluation method.
The magnetic recording medium using the surface-modified magnetic particles and the surface-modified filler according to the present invention has a coercive force value of preferably 39.8 to 318.3 kA/m (500 to 4,000 Oe), more preferably 43.8 to 318.3 kA/m (550 to 4,000 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm; hereinafter referred to merely as xe2x80x9csquarenessxe2x80x9d) of preferably 0.85 to 0.95, more preferably 0.86 to 0.95; a gloss of coating film of preferably 165 to 300%, more preferably 170 to 300%; a surface roughness Ra of coating film of preferably not more than 10.5 nm, more preferably 2.0 to 10.0 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of preferably 127 to 160, more preferably 128 to 160; a running durability of preferably not less than 23 minutes, more preferably not less than 24 minutes; and a scratch resistance of preferably Rank A or B, more preferably Rank A as measured by the below-mentioned evaluation method. In addition, the cyclohexanone dimer extraction amount is preferably not more than 49 mg/m2, more preferably not more than 39 mg/m2, still more preferably not more than 29 mg/m2 as measured by the below-mentioned evaluation method.
Also, in the case where in the consideration of high-density recording, the surface-modified magnetic particles produced by using magnetic acicular metal particles containing iron as a main component or magnetic acicular iron alloy particles as the raw particles, are used together with the ordinary filler, the obtained magnetic recording medium of the present invention has a coercive force value of preferably 63.7 to 278.5 kA/m (800 to 3,500 Oe), more preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm; hereinafter referred to merely as xe2x80x9csquarenessxe2x80x9d) of preferably 0.85 to 0.95, more preferably 0.86 to 0.95; a gloss of coating film of preferably 195 to 300%, more preferably 200 to 300%; a surface roughness Ra of coating film of preferably not more than 9.0 nm, more preferably 2.0 to 8.5 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of preferably 131 to 160, more preferably 132 to 160; a running durability of preferably not less than 25 minutes, more preferably not less than 26 minutes; and a scratch resistance of preferably Rank A or B, more preferably Rank A as measured by the below-mentioned evaluation method. In addition, the cyclohexanone dimer extraction amount is preferably not more than 50 mg/m2, more preferably not more than 40 mg/m2, still more preferably not more than 30 mg/m2 as measured by the below-mentioned evaluation method.
Further, in the case where in the consideration of high-density recording or the like, the surface-modified magnetic particles produced by using magnetic acicular metal particles containing iron as a main component or magnetic acicular iron alloy particles as the raw particles are used together with the surface-modified filler, the obtained magnetic recording medium of the present invention has a coercive force value of 63.7 to 278.5 kA/m (800 to 3,500 Oe), more preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm; hereinafter referred to merely as xe2x80x9csquarenessxe2x80x9d) of preferably 0.85 to 0.95, more preferably 0.86 to 0.95; a gloss of coating film of preferably 195 to 300%, more preferably 200 to 300%; a surface roughness Ra of coating film of preferably not more than 9.0 nm, more preferably 2.0 to 8.5 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of preferably 132 to 160, more preferably 133 to 160; a running durability of preferably not less than 26 minutes, more preferably not less than 27 minutes;; and a scratch resistance of preferably Rank A or B, more preferably Rank A as measured by the below-mentioned evaluation method. In addition, the cyclohexanone dimer extraction amount is preferably not more than 49 mg/m2, more preferably not more than 39 mg/m2, still more preferably not more than 29 mg/m2 as measured by the below-mentioned evaluation method.
The magnetic recording medium of the present invention may further comprise a non-magnetic undercoat layer disposed between the surface of the non-magnetic base film and the magnetic recording layer, which comprises the surface-modified non-magnetic particles or the ordinary non-magnetic particles and a binder resin.
The non-magnetic undercoat layer of the present invention comprises the surface-modified non-magnetic particles or the ordinary non-magnetic particles and a binder resin.
As the binder resin of the non-magnetic undercoat layer, there may be used the same binder resins as those used for forming the magnetic recording layer.
The amount of the surface-modified non-magnetic particles or the ordinary non-magnetic particles contained in the non-magnetic undercoat layer is preferably 5 to 2,000 parts by weight, more preferably 100 to 1,000 parts by weight based on 100 parts by weight of the binder resin contained in the non-magnetic undercoat layer.
When the amount of the surface-modified non-magnetic particles or the ordinary non-magnetic particles is less than 5 parts by weight, it may be difficult to continuously disperse the surface-modified non-magnetic particles or the ordinary non-magnetic particles in the non-magnetic undercoat layer because of a too small amount of the surface-modified non-magnetic particles or the ordinary non-magnetic particles in a non-magnetic coating composition, resulting in deteriorated surface smoothness of the obtained coating film. When the amount of the surface-modified non-magnetic particles or the ordinary non-magnetic particles is more than 2,000 parts by weight, it may be difficult to sufficiently disperse the surface-modified non-magnetic particles or the ordinary non-magnetic particles in the non-magnetic coating composition because of a too large amount of the surface-modified non-magnetic particles or the ordinary non-magnetic particles as compared to the amount of the binder resin. As a result, it may be difficult to obtain a coating film having a sufficiently smooth surface. In addition, since such a large amount of the surface-modified non-magnetic particles or the ordinary non-magnetic particles cannot be sufficiently bound together by the binder resin, the obtained coating film tends to become brittle.
Meanwhile, the non-magnetic undercoat layer may contain, if required, known additives for an ordinary magnetic recording medium such as lubricants, abrasives and antistatic agents in an amount of about 0.1 to about 50 parts by weight based on 100 parts by weight of the binder resin.
The non-magnetic undercoat layer using the surface-modified non-magnetic particles used in the present invention has a gloss of coating film of preferably 176 to 300%, more preferably 180 to 300%, still more preferably 184 to 300%; a surface roughness Ra of coating film of preferably 0.5 to 11.0 nm, more preferably 0.5 to 10.5 nm; and a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) (as strength of coating film) of preferably 128 to 160, more preferably 130 to 160. In addition, the cyclohexanone dimer extraction amount of the non-magnetic undercoat layer of the present invention is preferably not more than 40 mg/m2, more preferably not more than 30 mg/m2, still more preferably not more than 20 mg/m2 as measured by the below-mentioned evaluation method.
The magnetic recording layer formed on the non-magnetic undercoat layer has a thickness of usually 0.01 to 5 xcexcm, preferably 0.05 to 1 xcexcm. When the thickness of the magnetic recording layer is less than 0.01 xcexcm, it may be difficult to form a uniform coating film, resulting in problems such as coating unevenness or the like. When the thickness of the magnetic recording layer is more than 5 xcexcm, the obtained magnetic recording medium may fail to show an aimed electromagnetic performance because of adverse influence of diamagnetic field.
The magnetic recording medium of the present invention which has the non-magnetic undercoat layer disposed between the non-magnetic base film and the magnetic recording layer, and is produced by using the surface-modified non-magnetic particles, the ordinary magnetic particles and the ordinary filler, respectively, has a coercive force value of preferably 39.8 to 318.3 kA/m (500 to 4,000 Oe), more preferably 43.8 to 318.3 kA/m (550 to 4,000 Oe); a squareness of preferably 0.85 to 0.95, more preferably 0.86 to 0.95; a gloss of coating film of preferably 170 to 300%, more preferably 175 to 300%; a surface roughness Ra of coating film of preferably not more than 10.0 nm, more preferably 2.0 to 9.5 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of preferably 128 to 160, more preferably 129 to 160; a running durability of preferably not less than 23 minutes, more preferably not less than 24 minutes; and a scratch resistance of preferably Rank A or B, more preferably Rank A as measured by the below-mentioned evaluation method. In addition, the cyclohexanone dimer extraction amount is preferably not more than 45 mg/m2, more preferably not more than 35 mg/m2, still more preferably not more than 25 mg/m2 as measured by the below-mentioned evaluation method.
The magnetic recording medium of the present invention which has the non-magnetic undercoat layer disposed between the non-magnetic base film and the magnetic recording layer, and is produced by using the surface-modified non-magnetic particles, magnetic acicular metal particles containing iron as a main component or magnetic acicular iron alloy particles, and the ordinary filler, respectively, in the consideration of high-density recording, has a coercive force value of preferably 63.7 to 278.5 kA/m (800 to 3,500 Oe), more preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm; hereinafter referred to merely as xe2x80x9csquarenessxe2x80x9d) of preferably 0.85 to 0.95, more preferably 0.86 to 0.95; a gloss of coating film of preferably 195 to 300%, more preferably 200 to 300%; a surface roughness Ra of coating film of preferably not more than 9.0 nm, more preferably 2.0 to 8.5 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of preferably 132 to 160, more preferably 133 to 160; a running durability of preferably not less than 26 minutes, more preferably not less than 27 minutes; and a scratch resistance of preferably Rank A or B, more preferably Rank A as measured by the below-mentioned evaluation method. In addition, the cyclohexanone dimer extraction amount is preferably not more than 45 mg/m2, more preferably not more than 35 mg/m2, still more preferably not more than 25 mg/m2 as measured by the below-mentioned evaluation method.
The magnetic recording medium of the present invention which has the non-magnetic undercoat layer disposed between the non-magnetic base film and the magnetic recording layer, and is produced by using the surface-modified non-magnetic particles, the surface-modified magnetic particles and the ordinary filler, respectively, has a coercive force value of preferably 39.8 to 318.3 kA/m (500 to 4,000 Oe), more preferably 43.8 to 318.3 kA/m (550 to 4,000 Oe); a squareness of preferably 0.85 to 0.95, more preferably 0.86 to 0.95; a gloss of coating film of preferably 175 to 300%, more preferably 180 to 300%; a surface roughness Ra of coating film of preferably not more than 9.5 nm, more preferably 2.0 to 9.0 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of preferably 129 to 160, more preferably 130 to 160; a running durability of preferably not less than 24 minutes, more preferably not less than 25 minutes; and a scratch resistance of preferably Rank A or B, more preferably Rank A as measured by the below-mentioned evaluation method. In addition, the cyclohexanone dimer extraction amount is preferably not more than 44 mg/m2, more preferably not more than 34 mg/m2, still more preferably not more than 24 mg/m2 as measured by the below-mentioned evaluation method.
The magnetic recording medium of the present invention which has the non-magnetic undercoat layer disposed between the non-magnetic base film and the magnetic recording layer, and is produced by using the surface-modified non-magnetic particles, the surface-modified magnetic particles and the surface-modified filler, respectively, has a coercive force value of preferably 39.8 to 318.3 kA/m (500 to 4,000 Oe), more preferably 43.8 to 318.3 kA/m (550 to 4,000 Oe); a squareness of preferably 0.85 to 0.95, more preferably 0.86 to 0.95; a gloss of coating film of preferably 175 to 300%, more preferably 180 to 300%; a surface roughness Ra of coating film of preferably not more than 9.5 nm, more preferably 2.0 to 9.0 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of preferably 131 to 160, more preferably 132 to 160; a running durability of preferably not less than 25 minutes, more preferably not less than 26 minutes; and a scratch resistance of preferably Rank A or B, more preferably Rank A as measured by the below-mentioned evaluation method. In addition, the cyclohexanone dimer extraction amount is preferably not more than 43 mg/m2, more preferably not more than 33 mg/m2, still more preferably not more than 23 mg/m2 as measured by the below-mentioned evaluation method.
The magnetic recording medium of the present invention which has the non-magnetic undercoat layer disposed between the non-magnetic base film and the magnetic recording layer, and is produced by using the surface-modified non-magnetic particles, the surface-modified magnetic particles obtained using magnetic acicular metal particles containing iron as a main component or magnetic acicular iron alloy particles as the raw particles, and the surface-modified filler, respectively, in the consideration of high-density recording or the like, has a coercive force value of preferably 63.7 to 278.5 kA/m (800 to 3,500 Oe), more preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm; hereinafter referred to merely as xe2x80x9csquarenessxe2x80x9d) of preferably 0.85 to 0.95, more preferably 0.86 to 0.95; a gloss of coating film of preferably 200 to 300%, more preferably 205 to 300%; a surface roughness Ra of coating film of preferably not more than 8.5 nm, more preferably 2.0 to 8.0 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of preferably 133 to 160, more preferably 134 to 160; a running durability of preferably not less than 27 minutes, more preferably not less than 28 minutes; and a scratch resistance of preferably Rank A or B, more preferably Rank A as measured by the below-mentioned evaluation method. In addition, the cyclohexanone dimer extraction amount is preferably not more than 43 mg/m2, more preferably not more than 33 mg/m2, still more preferably not more than 23 mg/m2 as measured by the below-mentioned evaluation method.
Next, the process for producing the surface-modified particles used in the present invention is described.
In the present invention, the surface-modified particles comprising the raw particles coated with the organosilane compound produced from the silane monomer can be produced by mechanically mixing and stirring the raw particles with the organosilane compound produced from the silane monomer, or by mechanically mixing and stirring the raw particles and the organosilane compound produced from the silane monomer while spraying the organosilane compound produced from the silane monomer onto the raw particles. In these methods, substantially whole amount of the organosilane compound produced from the silane monomer can be coated onto the surface of the raw particles.
In order to uniformly coat the surface of the raw particles with the organosilane compound produced from the silane monomer, it is preferred that the raw particles are previously deagglomerated by using a pulverizer.
As the apparatus for mixing and stirring the raw particles and the silane monomer, there may be suitably used apparatuses capable of applying a shear force to the particles, more preferably apparatuses capable of conducting the application of shear force, spatula-stroking force and compression force at the same time. As such apparatuses, there may be exemplified wheel-type kneaders, ball-type kneaders, blade-type kneaders, roll-type kneaders or the like. Among these apparatuses, wheel-type kneaders are preferred.
Specific examples of the wheel-type kneaders may include an edge runner (equal to a mix muller, a Simpson mill or a sand mill), a multi-mull, a Stotz mill, a wet pan mill, a Conner mill, a ring muller or the like. Among these kneaders, an edge runner, a multi-mull, a Stotz mill, a wet pan mill and a ring muller are preferred, and an edge runner is more preferred. Specific examples of the ball-type kneaders may include a vibrating mill or the like. Specific examples of the blade-type kneaders may include a Henschel mixer, a planetary mixer, a Nawter mixer or the like. Specific examples of the roll-type kneaders may include am extruder or the like.
In order to coat the surface of the raw particles with the organosilane compound produced from the silane monomer as uniformly as possible, the conditions of the above mixing and stirring treatment may be appropriately controlled such that the linear load is preferably 19.6 1,960 N/cm (2 to 200 Kg/cm), more preferably 98 to 1,470 N/cm (10 to 150 Kg/cm), still more preferably 147 to 980 N/cm (15 to 100 Kg); the treating time is preferably 5 to 180 minutes, more preferably 10 to 120 minutes; and the stirring speed is preferably 2 to 2,000 rpm, more preferably 5 to 1,000 rpm, still more preferably 10 to 800 rpm.
Meanwhile, when readily oxidizable magnetic particles such as magnetite particles, magnetic acicular metal particles containing iron as a main component and magnetic acicular iron alloy particles are used, it is preferred that an inert gas such as N2 is filled in the mixer upon the treatment.
The amount of the silane monomer added is preferably 0.43 to 145.5 parts by weight based on 100 parts by weight of the raw particles to be treated. When the amount of the silane monomer added is less than 0.43 part by weight, it may be difficult to inhibit the production of cyclohexanone diner in the obtained particles. When the amount of the silane monomer added is more than 145.5 parts by weight, the effect of inhibiting the production of cyclohexanone dimer is already saturated and, therefore, the addition of such a large amount of the silane monomer is unnecessary and meaningless.
Meanwhile, the thus surface-treated particles may be preferably subsequently dried or heat-treated. In this case, the drying or heat-treating temperature is preferably 40 to 150xc2x0 C.m more preferably 60 to 120xc2x0 C., and the drying or heat-treating time is preferably 10 minutes to 12 hours, more preferably 30 minutes to 3 hours. When the drying or heat-treating temperature is more than 150xc2x0 C., the silane monomer tends to be vaporized or deteriorated in quality, so that it may be difficult to obtain aimed surface-modified particles.
The surface-modified particles obtained by dry-mixing method, i.e., by mechanically mixing and stirring the raw particles and the silane monomer, or by mechanically mixing and stirring the raw particles and the silane monomer while spraying the silane monomer onto the raw particles, have a resin adsorption of usually not less than 70% and a fatty acid adsorption of usually not more than 16 mg/g. More specifically, the surface-modified magnetic particles exhibit a resin adsorption of usually not less than 70% and a fatty acid adsorption of usually not more than 16 mg/g; the surface-modified filler exhibits a resin adsorption of usually not less than 70% and a fatty acid adsorption of usually not more than 7 mg/g; and the surface-modified non-magnetic particles exhibit a resin adsorption of usually not less than 70% and a fatty acid adsorption of usually not more than 15 mg/g.
Prior to being coating with the organosilane compound produced from the silane monomer, the raw particles may be previously coated with at least one coating material selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon.
The above coating material may be applied onto the raw particles as follows. That is, an aluminum compound, a silicon compound or both the compounds are added to a water suspension obtained by dispersing the raw particles in water. The resultant suspension is mixed and stirred and, if required, the pH of the suspension is then adjusted to an appropriate value, thereby coating the surface of the raw particles with at least one coating material selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon. Then, the thus obtained coated particles are filtered out, washed with water, dried and then pulverized. Further, the obtained particles may be subjected to deaeration, compaction or other suitable treatments.
As the aluminum compounds, there may be exemplified aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride and aluminum nitrate, alkali aluminates such as sodium aluminate, or the like.
As the silicon compounds, there may be exemplified water glass #3, sodium orthosilicate, sodium metasilicate or the like.
Next, the processes for producing respective magnetic recording medium according to the present invention are described.
The magnetic recording medium of the present invention can be produced by applying a magnetic coating composition containing either surface-modified magnetic particles or ordinary magnetic particles, the surface-modified filler or ordinary filler, a binder resin and a solvent onto a non-magnetic base film to form a coating layer thereon; and subjecting the coating layer to magnetic orientation, calender treatment and then curing, by ordinary methods.
Also, the magnetic recording medium of the present invention can be produced by applying a non-magnetic coating composition containing either surface-modified non-magnetic particles or ordinary non-magnetic particles, a binder resin and a solvent onto a non-magnetic base film and drying the resultant coating layer to form a non-magnetic undercoat layer; applying a magnetic coating composition containing either surface-modified magnetic particles or ordinary magnetic particles, either surface-modified filler or ordinary filler, a binder resin and a solvent onto the non-magnetic undercoat layer, wherein at least one selected from the group consisting of the surface-modified non-magnetic particles, the surface-modified magnetic particles and surface-modified filler is used; and subjecting the resultant coating layer to magnetic orientation, calender treatment and then curing to form a magnetic recording layer, by ordinary methods.
The kneading and dispersing of the magnetic coating composition and the non-magnetic coating composition may be performed using, for example, kneaders such as twin-screw kneader, twin-screw extruder, press kneader, twin-roll mill, triple-roll mill, or dispersing apparatuses such as ball mill, sand grinder, attritor, disper, homogenizer and ultrasonic dispersing device.
The coating of the magnetic coating composition and the non-magnetic coating composition may be conducted using a gravure coater, a reverse roll coater, a slit coater, a die coater or the like. The obtained coating film may be magnetically orientated using opposed magnets, solenoid magnet or the like.
As the solvents, there may be used methyl ethyl ketone, toluene, cyclohexanone, methyl isobutyl ketone, tetrahydrofuran, a mixture of these solvents or the like.
The total amount of the solvent used for the magnetic coating composition is 65 to 1,000 parts by weight based on 100 parts by weight of the particles. When the amount of the solvent used is less than 65 parts by weight, the viscosity of the coating composition prepared therefrom becomes too high, thereby making it difficult to apply the coating composition. On the other hand, when the amount of the solvent used is more than 1,000 parts by weight, the amount of the solvent volatilized during the formation of the coating film becomes too large, thereby rendering the coating process industrially disadvantageous.
The point of the present invention is that the surface-modified particles obtained by coating the surface of the raw particles with the organosilane compound produced from the silane monomer by dry-mixing method, are capable of not only inhibiting the production of cyclohexanone dimer in a coating composition upon production of magnetic recording medium, but also exhibiting a high resin adsorption and a low fatty acid adsorption.
The reason why the production of cyclohexanone dimer in a coating composition can be inhibited by using the surface-modified particles is not clear. However, it is considered that by coating the organosilane compound produced from the silane monomer on the surface of the raw particle, the surface activity of the particles can be reduced.
Hitherto, when the particles are surface-treated for improving the dispersibility in a coating composition upon the production of a magnetic recording medium, the increased resin adsorption thereof is accompanied with increase in fatty acid adsorption, while the reduced fatty acid adsorption thereof leads to reduction in resin adsorption. Thus, the conventional surface treatment has failed to satisfy both the increased resin adsorption and the reduced fatty acid adsorption at the same time. More specifically, in the case of acidic surface treatment (for example, surface-treatment with a silicon-containing coating material), although the fatty acid adsorption of the obtained particles can be reduced due to the acidity of fatty acid, the resin adsorption thereof is also reduced since many of resins used in the coating composition have acidic functional groups. Therefore, the acidic surface treatment has failed to satisfy both the increased resin adsorption and the reduced fatty acid adsorption at the same time. In addition, in the case of basic surface treatment (for example, surface-treatment with an aluminum-containing coating material), although the resin adsorption of the obtained particles can be increased, the fatty acid adsorption is also increased, thereby failing to satisfy both the increased resin adsorption and the reduced fatty acid adsorption at the same time. On the other hand, in the present invention, although the reason why the fatty acid adsorption can be reduced while keeping the high resin adsorption, is not clearly known, it is considered that by coating the surface of the raw particles with the organosilane compound produced from the silane monomer according to the process of the present invention, it is possible to effectively and uniformly form an organosilane compound coat on the surface of the particles.
The magnetic recording medium using the surface-modified particles according to the present invention has a high coating film strength and an excellent running durability. The reason why the magnetic recording medium of the present invention can exhibit a high coating film strength, is considered to be that the production of cyclohexanone diner in the coating composition can be prevented by using the surface-modified particles therein as described above. Also, the reason why the magnetic recording medium of the present invention can exhibit an excellent running durability, is considered to be that the surface-modified particles can be enhanced in resin adsorption and reduced in fatty acid adsorption as described above.
Thus, since the surface-modified particles used in the present invention can effectively prevent the production of cyclohexanone dimer in a magnetic or non-magnetic coating composition, the obtained magnetic recording medium can exhibit excellent coating film strength and running durability. Therefore, the surface-modified particles used in the present invention are suitably used as materials for a high-density magnetic recording medium.
Also, the magnetic recording medium according to the present invention can exhibit excellent coating film strength and running durability by using the surface-modified particles therein and, therefore, suitable as a high-density magnetic recording medium.